1. Field of the Invention
This invention relates in general to the field of microelectronics, and more particularly to a mechanism for enabling and sustaining a multi-processor environment on a bus that requires active control of bus termination impedances.
2. Description of the Related Art
Many present day bus architectures provide only for a point-to-point bus interface between two devices such as a microprocessor and its corresponding memory controller. In addition to providing only for a point-to-point interface, the architectures also require that the microprocessor (or other device) provide termination impedance control circuits within to dynamically adjust a termination impedance on the point-to-point bus, where the value of the impedance is generally selected to match the characteristic impedance of the bus itself.
In many applications, the value of the impedance is communicated to the device by coupling a precision resistor to an I/O pin on the device. Accordingly, the device provides drivers on-die that are configured to drive the point-to-point bus at the selected impedance value and at voltage levels in accordance with the bus specifications. These drivers provide for a properly terminated transmission line that has no reflections.
And while the point-to-point bus is effective for the case where only two devices communicate over the bus, the present inventors have noted for certain application areas such as a multi-processor application, more than one device may be required. In these applications, perhaps one to four processors are required to interface in parallel to a memory controller over a bus as described above.
But conventional bus architectures are limited because they require active impedance control under the presumption of fixed number of processors. For instance, the addition of another device on the bus described above would result in any given device seeing an effective termination impedance that is developed by the parallel termination impedances of the other devices on the bus and, accordingly, driving I/O signals into this effective pull-up termination impedance would result in reflections, ringing, timing displacements, and other disadvantages.
Consequently, the present inventors have observed that it is highly desirable to provide for inter-operation of a variable number of devices over a bus that requires active impedance control.
In addition, the present inventors have noted a need in the art for enabling a multi-processor environment over an actively controlled bus.